Method for pretensioning and anchoring reinforcements of concrete



Jan. 3. 1956 M. BIRKENMAIER ET AL 2,728,978

METHOD FOR PRETENSIONING AND ANCHORING REINFORCEMENTS OF CONCRETE Filed March 8, 1951 2 Sheets-Sheet 1 INVEN TOR MAX BIRKENMAIER ANTONIO BRANDESTINI MIRKO ROBIN ROS 0nd KURT VOGT 16 tab-all.

Jan. 3, 1956 M. BIRKENMAIER ET AL 2,728,978

METHOD FOR PRETENSIONING AND ANCHORING REINFORCEMENTS OF CONCRETE Filed March 8, 1951 2 Sheets-Sheet 2 4 E i' fw .:'-1 1 1' 1" R\ /NVENTOR:

1 MAX BIRK'ENMAIER 7 ANTONIO BRANDESTINI 233 MIRKO ROBIN ROS B and KURT VOGT At-t a vneqs United States Patent METHOD FOR PRETENSIONING-AND ANCHOR- IN G REINFORCEMENTS OF CONCRETE Max Birkenmaier, Antonio Brandestini, and Mirko Robin Ros, Zurich, and-Kurt Vogt, Beinwilam See, Switzerland Application March 8, 1951,.'Serial No. 214,611 Claims priority, application Switzerland March 10, 1950 2 Claims. (Cl. 29-452) Our present invention relates .to an improved method for pre-tensioning and anchoring reinforcements of *concrete structures or structural concrete elements. Pretensioned reinforcements consistingof steel wires of high tensile strength are used, above all, 'for:the fabrication of prestressed structural concrete elements and .forithe erection of prestressed concrete structures.

At present there are .known :mainly two methods and the appurtenant means for fabricating pre-tensioned reinforcements involving terminal anchors. In the first method, the single reinforcing wires (cables) at their ends extend along the wall of a (conical) openingin the structure to be prestressed. The wires are tensioned by special means at the circumference of which the wires have to be secured by wedge means, whereupon at least one element, e. g. a cone, is'forced intosaid opening so as to give origin to sufficient friction for preventing the wires from slipping.

Such first known method has the disadvantage that it is not certain whether all the single wires are 'tensioned at the same rate after having been anchored. In other words, it is questionable whether the simultaneous anchorage of more than two wires (as a rule .8-20 and more such wires) by virtue of wedge action is positive or whether not one or the other wire slip, which is readily conceivable. A further "disadvantage of such first known method is that it is practically impossible to check or ascertain the anchorage and the actual rate of pro-tension of the wires. The pre-tensioning apparatus, moreover, is comparatively heavy and complicated.

In the second known method, the individual reinforcing wires (a reinforcement unit-in such case comprises a plurality of .wires which run in rectangular arrangement-e. g. 4 by 8:32 wires-ata certain spacing from and parallel toeach other) at their ends pass in pairs through the rectangularopenings of a steel anchoring means. The wires by specialrneans and in pairs are successively pre-tensioned and anchored in pairs by gripping action by means of a steel wedge of rectangular cross-section.

The disadvantages of such second known method are mainly of economic nature in that the terminal anchorage used is comparatively expensive. Furthermore, the pair- Wise tensioning of the wires requires substantially more time.

It also is known in the art to provide single wires or strands with sleeves which are positively secured to the wires or strands. The said sleeve is connected to a pretensioning apparatus in order to pretension the wires. It is a disadvantage of such known method that the said sleeves are expensive and that only small-diameter strands may be secured thereto so that the maximum tension to be applied to a cable is limited to 304() tons.

The general object of our present invention is to avoid the said technical and economical disadvantages. Our invention is characterised in that the wires to be pretensioned are anchored at their ends to a common chored thereto by upsetting-their ends. The Wires are -pre tensioned with the aid of a tensioning means which engages the anchor, :whereupon the anchor is .propped 'up against the structure.

The advantages of the anchorage provided by our present invention are as follows:

It is economical, as-it may bemade with simple means. The cylindrical anchors may be .produced relatively cheaply on automatic lathes, 'orbycasting or pressing, .whilst the anchoring "heads may =be upset in a simple way and manner with -'the aid of a means developed by the :applicants. A"further advantage from a commercial point of view is afiordedby the use of wire ropes which may be'made in=a simple way and manner.

It maybe readily adapted to low aswell as high pre-tension forces. By combining any desired number of wires .in asingle anch r head, apre-tension force of several thousand tonsmay be used.

The'pre-tension :may be controlled step-by-step, and the pre-tensioning force may be ascertained 'or .checked any time by re-engaging the -jack in the anchor heads.

All the cables may be tensioned simultaneously, subjecting the sameto maximum forces.

The anchorage -is technical1y positive, since it is not based on frictional or gripping forces. In'the two known methods mentioned above,.the wires first are anchored against the pre-tensioning means, and a'second anchorage is provided subsequent to the-pre-tensioning operation, which second anchorage is propped on the concrete. Any part-failure of said second=-anchorage 'is extremely hard to ascertain. In themethod disclosed herein, however, only one anchorage -is used, and the operation thereof may be accurately controlled during the pre-tensioning operation. The-positive operationof the anchorage -also may I be 'ascertainedby applying -'an excess tension.

Since the cables-may'be pre-tensioned collectively and atboth :ends, :and 'since the contemplated tension may .be exceeded,=thet1oss-of tensiondue'to 'f'riction maybe eliminated 'or at least substantially reduce'dby a simul- Several forms of apparatus for carrying out the method disclosed herein, are shown in the accompanying drawing in which Fig. 1 shows the reinforcement and a first form of the apparatus in a side view and partly in section,

Figs. 2 and 3 are cross-sections on the lines II-II and III-III of Fig. 1, respectively,

Fig. 4 shows, in a larger scale, a longitudinal section through the anchor, draw-bar, propping nut, and the prop,

Fig. 5 is a section on the line V-V of Fig. 4,

Fig. 6 shows a longitudinal section through the pretensioned reinforcement anchored, through anchor and propping nut, on the structure,

Fig. 7 is a longitudinal section through the pre-tensioned reinforcement according to a second form.

Fig. 8 shows a third form, the most simple form of our present method.

Figs. 9 to 12 show further forms of the anchor heads.

In the form of apparatus shown in Fig. 1, there are provided a jack 1, having a hydraulic piston 2, a pump 3 and a manometer 4. The piston has a continuous axial opening 5 through which passes a draw bar 6. The latter at its one end is secured to a cylindrical, externally threaded anchor 7, whilst its other end bears on the jack 1 through a nut 8. At the foot of jack 1 is inserted a bearing element 9 which is propped against a plate 10.

The reinforcement is pre-tensioned and anchored as follows, having reference to Fig. l: The wires 13, which are disposed in an envelope 11, 12, at their ends are anchored (in the manner described) on the common anchor 7. The complete reinforcement (cable) Consisting of a plurality of wires 13, the envelopes 11 and 12, and the two anchors 7 (one at each end), is embedded in the structure to be erected, for example in a concrete bridge. The draw bar 6 is secured to the anchor 7, e. g. screwed thereto, whereupon the nut 8, the bearing element 9 and the jack 1 are brought into position. The nut 8 is mounted on the bar 6 and screwed down against the end 2' of the jack.

The pump 3 then is operated by means of an arm 15, and the piston 2 pulls the bar 6 and the anchor '7 outwardly, i. e. to the right in Fig. 1. The wires 13 thereby are tensioned.

In order to anchor the reinforcement, a bearing nut 16, which is screwed on the external threads of the anchor 7, is moved towards the plate 10 by means of a tool inserted through a window 9' in the element 9.

The draw bar 6, the jack 2 and the element 9 then may be removed, as shown in Fig. 6, and the reinforcement is in the pre-tensioned condition. Cement mortar or another suitable material then is forced into a hollow space 17 between the envelope 11, 12 and the wires 13.

When anchoring the upset wire heads 14 in the anchor 7, the latter will be relatively short. In the case of a long path to be covered during the pre-tensioning operation, the nut 16 (Fig. 4) will be mounted on the anchor 7 after the latter has emerged from the structure.

In Fig. 7 is shown an arrangement for anchoring the reinforcement on the structure without the nut 16. The anchor 7' has no external threads, and is temporarily held in its final position (for example by momentarily anchoring the draw bar 6) until the mortar forced into the hollow space 17, has set.

Any number of reinforcing wires may be employed, e. g. only one as shown in Fig. 8. A steel wire 13 is passed through a cylindrical anchor provided with a bore, and its end 14 is upset to form an anchor head. After the wire has been pre-tensioned as desired, the anchor head is spaced a distance d from the end of the article 18 which is to be prestressed. By tightening the nut 16 in Fig. 9, or by mounting a shim 19 in Fig. 10, the anchor head is held at the desired distance. When a plurality of wires are used, the means shown in Fig. 1 is used.

The pre-tensioning operation may be carried out from both ends simultaneously, which procedure is very important from the point of view of decreasing the friction losses.

Further substantial advantages of our present invention are as follows. The reinforcement assembly-40mprising wires, a flexible sheath or envelope, and the anchors-may be delivered as a unit on the building site, having been tested in the shop. It is possible to check the rate of pre-tension at any time, provided of course that no mortar has been forced yet into the sheath or envelope of the reinforcement.

What we claim as new and desire to secure by Letters Patent, is:

l. A method of producing an anchorage for prestressed reinforcing wires for a concrete structure comprising inserting a plurality of reinforcing wires in openings provided in an anchor member, upsetting the ends of said wires so that the upset ends bear against said member, then applying a prestressing force to said anchor member whereby said reinforcing wires are uniformly prestressed and then anchoring said anchor member to said concrete structure.

2. A method of producing an anchorage for prestressed reinforcing wires for a concrete structure comprising reinforcing the outlet of the recess for said wires in the concrete structure, then inserting the ends .of a plurality of reinforcing wires in openings provided in an anchor member, upsetting the ends of said wires so that the upset ends bear against said member, then applying a prestressing force to said anchor member whereby said reinforcing wires are uniformly prestressed, then surrounding said wires with mortar while maintaining said force upon said anchor member, then enveloping said mortar with a metal envelope, then allowing said mortar to set and then releasing said force upon said anchor member whereby said mortar supports said anchor member in final position.

References fired in the tile of this patent UNITED STATES PATENTS 903,909 Steiner Nov. 17, l908 l,387,895 McBride Aug. 16, 1921 1,389,154 Newhouse Aug. 30, 1921 1,479,712 Hallet Jan. 1, 1924 1,500,204 Richard July 8, 1924 1,819,782 Martin Aug. 18, 1931 2,095,714 Pinaud Oct. 12, 1937 2,193,078 Schenk Mar. 12, 194: 2,332,032 Troiel is Oct. 19, 1943 2,554,755 Sechaud et a] May 29, 1951 2,609,586 Parry Sept. 9, 1952 2,637,895 Blaton May 12, 1953 

